The invention relates generally to optical filters and, more particularly, to minimizing loss and ripple in such filters.
Many systems, such as wavelength division multiplexing (WDM), require filters with a relatively flat and smooth-topped spectral response. The prior art has shown that one can construct a filter with a flat spectral response by cascading two waveguide grating routers (WGRs) [1] (Note in this specification, a reference to another document is designated by a number in brackets to identify its location in a list of references found in the Appendix) I focus here on filters that consist of two cascaded gratings, such as waveguide gratings, connected by two or more connecting optical elements, such as waveguides, (which we will sometimes refer to as xe2x80x9cback-to-back gratingsxe2x80x9d), such as shown in FIG. 1. Examples of such filters include wavelength cross connects [2] wavelength equalizers [3], and wavelength add/drops [8].
Such back-to-back gratings do not generally exhibit a flat spectral response, and thus there is a need for providing design criteria for minimizing the loss and ripple in the spectral response through such back-to-back gratings.
In accordance with the present invention, I describe a design technique for minimizing the loss and ripple in the spectral response through a pair of gratings, such as waveguide gratings, connected by an array of optical elements, such as waveguides. I achieve the smoothest spectral response for a given set of connecting optical elements by choosing the aperture size of the grating to fill the central Brillouin zone (i.e., central diffraction order) as determined by the connecting optical elements. Exactly filling the Brillouin zone with the grating minimizes the loss, and so is the preferred choice.
More particularly, for the case of waveguide gratings, my low-ripple and low-loss spectral response optical filter includes two cascaded waveguide grating routers connected by a set of waveguides, each waveguide including a phase shifter, switch, shutter, couplers, amplifiers, and/or mirror. Each waveguide grating router comprises a pair of star couplers interconnected by a waveguide grating, each waveguide grating router further characterized in that the number of grating arms in the waveguide grating is chosen so as to fill, not substantially more or substantially less, the central Brillouin zone of each star coupler, as determined by the set of connecting waveguides for which one desires a ripple-free and low-loss spectral response.
According to my method, a low-ripple and low-loss spectral response is produced for an optical filter formed from two cascaded waveguide grating routers connected by a set of waveguides, where each waveguide grating router comprises a pair of star couplers interconnected by a waveguide grating. The method includes the steps of (1) determining the set of connecting waveguides for which one desires a ripple-free and low-loss spectral response and (2) selecting the number of grating arms in the waveguide grating so as to fill, not substantially more or substantially less, the central Brillouin zone of each star coupler, as determined by the set of connecting waveguides.